1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines and printers, using electrophotography and electrostatic recording.
2. Description of the Related Art
In recent years, advances have been made in techniques for recording image information in full color, and the need for faster, less expensive, and clearer outputs of hard copies is increasing.
Various measures have been taken to ensure that existing electrophotographic image forming apparatuses can satisfy the above need.
For example, in order to satisfy the need for higher speed, some color image forming apparatuses use four image bearing members corresponding to four colors (hereinafter referred to as a xe2x80x9cfour drum typexe2x80x9d), and image forming processes including charging, exposure, and development are performed at respective stations for each color. This allows image formation to be performed four times as fast as in an apparatus having only one image bearing member.
Furthermore, by changing the fixing speed so as to respond to various types of transfer materials, image quality can be improved for different transfer materials. For example, the fixing speed is generally decreased when forming images on cardboard, OHP sheets, or the like.
In this case, the fixing speed can be changed, without changing the image forming speed in the image forming processes prior to fixing, by using a fixing conveyor belt.
Since the above structure tends to increase the overall size of the apparatus, another method has been proposed in which the image forming speed is changed in all the processes. This eliminates the necessity for a fixing conveyor belt and results in a reduction in the size of the main body of the image forming apparatus. Such size reduction also leads to cost reduction, and as a result, a compact image forming apparatus can be provided at low cost.
However, since all characteristics, such as the characteristics of a photoconductive member serving as an image bearing member, the developing characteristics, and the transfer characteristics, are changed in accordance with the image forming speed, the image output characteristics are also changed as a result.
For example, regarding the characteristics of the photoconductive member, even when the exposure level per unit area is fixed, the potential of the photoconductive member at a developing section is not fixed because the time when the photoconductive member rotates from the exposure section to the developing section is changed.
Similarly, the developing characteristics will change along with the image forming speed.
Changes in the characteristics in the respective image forming processes appear as a change in image output characteristics.
As described above, conventional image forming apparatuses have a drawback in that it is difficult to obtain an image having a desired image density due to a change in image output characteristics which result when the image forming speed is changed.
Accordingly, an object of the present invention is to provide an image forming apparatus in which the image forming speed is variable.
Another object of the present invention is to provide an image forming apparatus in which a high-gradation image can be obtained even when the image forming speed is varied.
In order to achieve the above objects, according to an aspect of the present invention, there is provided an image forming apparatus including: an image bearing member; image forming means for forming an electrostatic image on the image bearing member in accordance with an image signal; developing means for developing an electrostatic image formed on the image bearing member; transfer means for transferring a developed image on the image bearing member onto a transfer material; speed setting means for selectively setting a moving speed of the image bearing member; and gamma characteristic selection means for selecting a gamma conversion characteristic for converting the relationship between the image signal and an output density in accordance with the moving speed of the image bearing member.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.